How to track an object to a specific property in the Blender Game Engine
Simple change. The syntax for the "Steering" actuator calls for a target field, not a track _object field. So your new snippet looks like this:

def target_prop(cont):
self = cont.owner
objects = self.scene.objects

test = [o for o in objects if self["Cube"] in o]
test.sort(key=lambda o: self.getDistanceTo(o))

target, *_ = test

act_track, *_ = cont.actuators
act_track.target = target

for act in cont.actuators:
     cont.activate(act)


you can see the only change is line #11